1. Field of the Invention
The present invention relates to positive active material for use in sealed alkaline storage batteries, more particularly to the active material of a positive electrode which, when incorporated in alkaline storage batteries, can provide thereto a high discharge capacity that is sustained over a long period of successive charge-discharge cycles.
2. Related Art
The use of sintered-nickel electrode for a positive electrode as of a nickel-metal hydride or nickel-cadmium storage battery has been conventionally known in the art. Such a sintered-nickel electrode contains active material (nickel hydroxide) loaded in a sintered substrate made by sintering nickel powder into a perforated steel plate or the like.
In the fabrication of sintered-nickel electrodes, if a high loading of active material is to be sought, a highly-porous sintered substrate must be used. However, the increased porosity of sintered substrate leads to the increased tendency for the nickel active material to fall off, since sintering is only effective to provide weak bonds between nickel powders. It is accordingly difficult in practice to increase its porosity to 80% or higher. This has restrained the sintered-nickel electrode from enjoying high loading of nickel active material. Also, the sintered substance has small pore sizes generally of not exceeding 10 xcexcm. This has required. that a complex impregnation process be repeated several times in loading the sintered substrate with.nickel active material.
In view of the above, a nonsintered-nickel electrode has been recently proposed and put to practical use. The nonsintered-nickel electrode can be fabricated by applying a paste consisting of a mixture of nickel hydroxide active material and a binder, such as an aqueous solution of methyl cellulose, to a highly-porous conductive core so that the core is loaded with the active material. The high porosity of the conductive core, 95% or higher, not only enables high loading of active material, but also facilitates loading of active material into the conductive core.
However, in the fabrication of nonsintered-nickel electrodes, the use of conductive core having an increased porosity in pursuit of increased loading of active material reduces its ability to collect current, leading to the reduced utilization factor of active material.
In order to increase the active material utilization factor of nonsintered-nickel electrodes, the addition of cobalt hydroxide to nickel hydroxide to provide a mixed positive active material has been proposed (Japanese Patent Publication No. 61-49374 (1986)).
Other proposals include adding cobalt monoxide to nickel hydroxide (Japanese Patent Laying-Open No. 61-138458 (1986)) and adding powders of cobalt hydroxide and yttrium compound to nickel hydroxide powder (Japanese Patent Laying-Open No. 5-28992 (1993)).
While these techniques are effective in improving the utilization factor of positive active material, such nonsintered-nickel positive electrodes, when incorporated into a cell or battery, increase a discharge reserve of a negative electrode thereof to result in the failure to obtain a sufficient battery capacity.
For sealed nickel-hydrogen and nickel-cadmium storage batteries, a negative electrode is designed to have excess capacity compared to a positive electrode capacity so that the negative electrode has a portion left uncharged even after the positive electrode has been fully charged. Accordingly, the positive electrode begins to evolve an oxygen gas at a final stage of charge. A sealing condition is maintained by the action of the negative electrode that absorbs the oxygen gas evolving from the positive electrode.
The oxygen-absorbing reaction of the nickel-cadmium storage battery during overcharge can be illustrated as follows:
Negative electrode 2OHxe2x88x92xe2x86x921/2O2+H2O+2exe2x88x92
Positive electrode Cd+1/2O2+H2Oxe2x86x92Cd(OH)2
In this reserve balance at the negative electrode, a discharge reserve is produced when divalent cobalt compounds primarily contained in the positive electrode, i.e. CoO and Co(OH)2, metallic cobalt and a part of nickel hydroxide that undergoes an irreversible reaction (corresponding to discharge from 2.1xcx9c2.3 valence to 2.0 valence) are oxidized to their trivalent forms.
The quantity of electricity involved in such oxidation reactions is accumulated in the negative electrode to define the quantity of discharge reserve.
The discharge reserve, while may appear that it is not involved in the charge and discharge reactions, functions virtually to suppress a voltage drop at the negative electrode during the final stage of discharge and during the high-rate discharge so that the positive electrode capacity can be discharged to the final end. The discharge reserve is therefore an essential factor when designing a cell or battery.
However, the quantity of discharge reserve produced does not coincide with but mostly exceeds the necessary quantity, since it is produced secondarily as stated above. Accordingly, if the capacity is to be increased for the nickel-hydrogen and nickel-cadmium storage batteries, the discharge reserve must be reduced in quantity. From this point of view, a technique is disclosed, for example, in Japanese Patent Publication No. 8-24041 (1996), which subjects a mixture of nickel hydroxide and cobalt monoxide to an oxidation treatment with potassium peroxodisulfate, as an oxidizing agent, in an aqueous solution of potassium hydroxide so that only cobalt monoxide is converted to the form of xcex2-CoOOH for use as active material. Also, the use of nickel oxyhydroxide covered with cobalt oxyhydroxide, as well as the use of solid solution particles consisting primarily of nickel oxyhydroxide, respectively for active materials, have been proposed, for example, in Japanese Patent Laying-Open No. 10-74512 (1998).
It has been found from the investigations conducted by the present inventors that the use of such techniques, while certainly possible to reduce the discharge reserve, lowers the charge acceptance of a battery or cell as a result of the oxidation treatment to result in the failure to obtain a sufficient level of discharge capacity.
The present invention is directed toward solving the above-described problems, and its object is to provide positive active material which can improve charge acceptance of a positive electrode to result in the constitution of a sealed alkaline storage battery having a high discharge capacity.
The active material of the present invention is the positive active material for use in sealed alkaline storage batteries, which is obtainable by subjecting xcex2-nickel hydroxide, together with an additive, to an oxidation treatment with an oxidizing agent in an aqueous alkaline solution. The additive comprises at least one type selected from yttrium, gadolinium, erbium and ytterbium, and oxides, hydroxides, fluorides and chlorides thereof.
Subjecting the xcex2-nickel hydroxide, together with the additive, to an oxidation treatment in an aqueous alkaline solution, according to the present invention, results in the provision of a positive active material which, when incorporated into a battery, cannot only reduce the discharge reserve but also increase the oxygen overvoltage during charge to improve charge acceptance, leading to a high discharge capacity of the battery. Although an exact reason why such results can be obtained is not clear, the effectiveness of the positive active material is believed to result from the oxidation treatment that causes metallic ions contained in the additive to diffuse into the crystal structure of nickel hydroxide.
In the present invention, the xcex2-nickel hydroxide, prior to being subjected to an oxidation treatment, is preferably covered or mixed with at least one type selected from cobalt hydroxide, cobalt monoxide and a sodium-containing cobalt compound. When mixed with the nickel hydroxide, these cobalt compounds are provided preferably in the form of particles having sizes smaller than that of the nickel hydroxide. Since these compounds act as conducting agents, their presence on nickel hydroxide surfaces serves to increase the utilization factor of active material, thereby facilitating discharging thereof. The amount of cobalt compound used to cover or form a mixture with xcex2-nickel hydroxide is preferably in the range of 1-10% by weight, as reduced to the amount of cobalt atoms, with respect to the amount of xcex2-nickel hydroxide. If the amount falls below 1% by weight, the cobalt content of the active material may become too small to result in obtaining a sufficient effect of improving the utilization factor thereof. On the other hand, if the amount goes beyond 10% by weight, the xcex2-nickel hydroxide content of the active material may become relatively small to result in the failure to obtain a sufficient discharge capacity.
The amount of additive is preferably in the range of 0.1-5% by weight, as reduced to the amount of yttrium, gadolinium, erbium or ytterbium element, with respect to the amount of the xcex2-nickel hydroxide. If the amount falls below 0.1% by weight, the failure to increase the oxygen overvoltage to an sufficient level may result which leads to the failure to obtain a sufficient discharge capacity. On the other hand, if the amount goes beyond 5% by weight, the xcex2-nickel hydroxide content of the active material becomes relatively small, possibly resulting in the failure to obtain a sufficient discharge capacity.
In the present invention, the valence numbers of nickel atoms in the positive active material are preferably in the range of 2.1-3.4. If they are below 2.1, the sufficient reduction of discharge reserve may not result. On the other hand, if they exceed 3.4, the xcex3-NiOOH is produced to reduce a bulk density of nickel powder. This may result in the insufficient loading of active material in a substrate, leading to the failure to obtain a sufficient cell or battery capacity.
The positive electrode of the present invention, for use in sealed alkaline storage batteries, is obtained by loading the positive active material of the present invention into an electrically conductive core.
The sealed alkaline storage battery of the present invention includes the aforementioned positive electrode of the present invention, a negative electrode and an alkaline liquid electrolyte. Examples of useful negative electrodes include a zinc electrode, a cadmium electrode and a hydrogen storage alloy electrode.
The method of the present invention for fabrication of a positive electrode for use in sealed alkaline storage batteries includes the steps of mixing the active material of the present invention with a binder to provide a paste containing the active material and loading the paste into a conductive core.
The method of the present invention for preparation of active material includes a step of providing xcex2-nickel hydroxide and at least one additive selected from yttrium, gadolinium, erbium and ytterbium, and oxides, hydroxides, fluorides and chlorides thereof, and a step of subjecting the xcex2-nickel hydroxide and the additive to an oxidation treatment with an oxidizing agent in an aqueous alkaline solution.
In the preparing method of the present invention, the aqueous alkaline solution, for use in the oxidation treatment, preferably contains sodium hydroxide and/or potassium hydroxide. The aqueous alkaline solution is preferably provided in the concentration not exceeding 30% by weight. If the concentration exceeds 30% by weight, the xcex3-nickel oxyhydroxide may be selectively produced during the oxidation treatment to reduce the loading of resulting active material into the electrode, resulting in the failure to obtain a sufficient discharge capacity.
During the oxidation treatment, the aqueous alkaline solution is preferably maintained at a reaction temperature of 10-50xc2x0 C. If the reaction temperature is below 10xc2x0 C., the oxidation reaction may be retarded. On the other hand, if it exceeds 50xc2x0 C., the reduced discharge capacity may result which is conceivably explained by the following reason: The excessively high reaction rate causes the rapid and selective oxidation of secondary particle surfaces of nickel hydroxide to result in the conversion of xcex2-nickel hydroxide to the xcex3-nickel oxyhydroxide, via the xcex2-nickel oxyhydroxide. This change in crystal form causes the active material to fall off, leading to a lower loading thereof.
The oxidizing agent used in the oxidation treatment may preferably be of at least one type selected from the group consisting of sodium hypochlorite (NaClO4), sodium peroxodisulfate (Na2S2O8), hydrogen peroxide (H2O2) and potassium peroxodisulfate (K2S2O8). By the use of such oxidizing agents, the degree of oxidation reaction between the xcex2-nickel oxyhydroxide and the aforementioned additive can be suitably controlled to produce effective active materials.
The following embodiments and examples illustrate the practice of the present invention but are not intended to be limiting thereof. Various changes and modifications can be suitably made without departing from the scope of the present invention.